Many types of printing devices, including but not limited to printers, copiers, and facsimile machines, print by transferring a printing fluid onto a printing medium. These printing devices typically include a printing fluid supply or reservoir configured to store a volume of printing fluid. The printing fluid reservoir may be located remotely from the print head assembly (“off-axis”), in which case the fluid is transferred to the print head assembly through a suitable conduit, or may be integrated with the print head assembly (“on-axis”). Where the printing fluid reservoir is located off-axis, the print head assembly may include a small reservoir that is periodically refilled from the larger off-axis reservoir.
Some printing devices may include a printing fluid detector configured to produce an out-of-fluid signal when printing fluid in the print head assembly or printing fluid reservoir drops below a predetermined level. This signal may be used to trigger the printing device to stop printing, and also to alert a user to the out-of-fluid state. The user may then replace (or replenish) the printing fluid reservoir and resume printing.
Various types of printing fluid detectors are known. Examples include, but are not limited to, optical detectors, pressure-based detectors, resistance-based detectors and capacitance-based detectors. Capacitance-based printing fluid detectors may utilize a pair of capacitor plates positioned adjacent, but external, to the printing fluid. These detectors measure changes in the capacitance of the plates with changes in printing fluid levels. However, the changes in capacitance of these systems may be too small to easily distinguish the capacitance changes from background noise. Thus, it may be difficult to accurately determine a printing fluid level, resulting in the generation of false out-of-fluid signals, and/or the failure to generate out-of-fluid signals when appropriate. Furthermore, many capacitance- and resistance-based detectors may have difficulty distinguishing printing fluid from printing fluid froth, which is commonly found in a printing fluid reservoir after the reservoir is substantially emptied of printing fluid.